Process for preparing pellets of poly(trimethylene terephthalate)

ABSTRACT

The process creates a radial temperature gradient in an extruded strand in a direction normal to the direction of motion thereof (that is, normal to the longitudinal direction thereof) so that upon exiting the first quench region the surface of the strand has solidified while at least the preponderant portion of the interior of the strand remains above the cold crystallization temperature, T cc  Then, in the annealing region, heat from the warmer interior will be transferred to the cooler surface, causing the surface to heat up into the range of T cc , thereby inducing the surface to undergo crystallization. Once the surface has undergone crystallization, the strand is thoroughly quenched in a second quench region so that the strand will be ready for pelletization, accumulation or other handling steps.

This application claims benefit of U.S. Provisional 61/447,868, filedMar. 1, 2011, and U.S. Provisional 61/447,875, filed Mar. 1, 2011, whichare herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to melt processing of poly(trimethyleneterephthalate) into pellets suitable for future use in fiber production,injection molding, and other fabrication methods.

BACKGROUND OF THE INVENTION

Polytrimethylene terephthalate (PTT) is prepared by the polycondensationreaction of 1,3-propanediol with terephthalic acid or terephthalic acidesters. PTT polymer and copolymers thereof are finding ever increasingcommercial use in the preparation of fibers, fabrics, carpets, films,molded parts and the like.

Standard practice in the polymer art is to manufacture thermoplasticpolyesters in a “continuous polymerizer” extruding the produced moltenpolymer though an extruder equipped with a so-called “strand die” whichis simply a metal plate having one or more about ⅛-¼″ circularcross-section holes in it. A continuous strand, about 1/10-⅛″ indiameter, of molten thermoplastic polymer is extruded out of each holeof the strand die. Immediately upon discharge from the strand die, thehot strand is usually passed through a water quench region normallyconsisting of a water quench tank or a water spray chamber. The lengthof the quench region is largely dependent upon the linear rate ofextrusion of the strand—that is, how fast the strand is moving—and themelt temperature of the strand at the exit of the die. At the end of thequench region, the strand is separated from the water, and is directedto a cutter that cuts the strand into pellets about ⅛″ long.

This practice, developed for poly(ethylene terephthalate) (PET), hasbeen applied to poly(trimethylene terephthalate) (PTT). Because of therapid quenching of the strand, the resulting pellets tend to exhibitamorphous (that is, non-crystalline) surfaces. This is not a problem inPET because the glass transition temperature is sufficiently high thatthe amorphous surfaces remain solid until the pellet is further meltprocessed. PTT exhibits a glass transition temperature (T_(g)) of about45° C.—much lower than that of PET and other well-known polyesters suchas PBT or PEN. It has been found that when PTT pellets prepared by theprocess described supra are stored where the ambient temperature reachesabout 40° C., some degree of surface polymer flow may occur on amicroscopic scale, thereby causing stored pellets to stick together,forming aggregates that hinder the smooth flow of pellets duringprocessing. By inducing surface crystallization, pellets can be producedthat exhibit greatly reduced sticking and aggregation when subject torelatively high storage temperatures.

Hanniman et al., US2009/0057935, discloses a process for producing anon-adhering granule from a polyester above T_(g) by introducing agranulated material into cooling water at 80-110° C., where thegranulated material is prepared from a melt and fed to an underwatermelt cutter.

Duh et al., U.S. Pat. No. 6,297,315, discloses a process forcrystallizing PTT before pelletizing by immersing the quenched strandinto a water batch heated to 60-100° C. Duh teaches that best resultsare obtained by crystallization after pelletization. Further, accordingto Duh, a water temperature of 60° C. requires a 20 minutecrystallization time to achieve satisfactory results. This is because,as Duh shows, 60° C. is below the crystallization temperature of about69° C.

Nishiyama et al, JP7223221 (A) (abstract only), discloses a processsuitable for use with a polyester having a T_(g) less than 40° C. inwhich a molten strand is quenched, then subject to stretching to inducecrystallization therein, followed by pelletization.

Shelby et al., U.S. Pat. No. 6,159,406, discloses a process forcrystallizing difficult to crystallize polyester melts beforepelletizing by quenching the strand, and then stretching the strand bywinding on two sets of godets separated by a hot water bath set aboveT_(g) through which the strand passes.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a process comprising:

-   -   (a) preparing a molten strand of a poly(trimethylene        terephthalate) composition wherein the strand has a surface and        an interior;    -   (b) contacting the molten strand with water in a first quench        region for a sufficient period of time so that the surface of        the molten strand solidifies and at least a preponderant portion        of the interior remains above the cold crystallization        temperature of the poly(trimethylene terephthalate) composition,        thereby preparing a surface-solidified strand;    -   (c) separating the surface-solidified strand from the water;    -   (d) exposing the surface-solidified strand to ambient air in an        annealing region for a sufficient period of time to induce cold        crystallization on the surface of the strand producing a        surface-crystallized strand; and    -   (e) contacting the surface-crystallized strand with water in a        second quench region for a sufficient period of time to solidify        the interior of the strand.

In another aspect, the invention provides an apparatus comprising

-   -   an extrusion means for extruding a molten polymer strand;    -   a pelletizer; and,    -   a trough disposed to convey a polymer strand from the extrusion        means to the pelletizer; the trough comprising        -   an interior bottom surface;        -   a water dispensing means disposed to provide a layer of            water on a first portion of the interior bottom surface that            is proximate to the extrusion means;        -   a mesh or perforated surface disposed in the trough            downstream from the water dispensing means, disposed to            permit the separation of water from a water-immersed strand            incident upon it; and,        -   a second portion of the interior bottom surface downstream            from that mesh or perforated surface, disposed to contain a            layer of water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a illustrates a side longitudinal cross-sectional view of a troughin an embodiment of the invention wherein the trough comprises aplurality of perforations in the interior bottom surface thereof.

FIG. 1b illustrates a bottom view of the trough shown in FIG. 1a ,showing the plurality of perforations downstream from the first portion,and a large drainage hole in the furthest downstream portion.

FIG. 2 illustrates a side longitudinal cross-sectional view of thetrough in an embodiment of the invention wherein the trough interiorbottom does not have perforations downstream from the first portion ofthe trough so that water introduced at the portion closest to the stranddie can flow unimpeded to the drainage hole at the end of the trough.Instead, in this embodiment, the trough is provided with a raised wiremesh portion that is disposed just above the water.

FIG. 3 illustrates a representation of an embodiment of the apparatusand process of the invention that was employed in the Examples, infra.

DETAILED DESCRIPTION OF THE INVENTION

When a range of numerical values is provided herein, it is intended toencompass the end-points of the range unless specifically statedotherwise. Numerical values used herein have the precision of the numberof significant figures provided, following the standard protocol inchemistry for significant figures as outlined in ASTM E29-08 Section 6.For example, the number 40 encompasses a range from 35.0 to 44.9,whereas the number 40.0 encompasses a range from 39.50 to 40.49. In oneaspect the present invention provides a process comprising:

-   -   (a) preparing a molten strand of a poly(trimethylene        terephthalate) composition wherein the strand has a surface and        an interior;    -   (b) contacting the molten strand with water in a first quench        region for a sufficient period of time so that the surface of        the molten strand solidifies and falls below the cold        crystallization temperature while at least a preponderant        portion of the interior remains above the cold crystallization        temperature of the poly(trimethylene terephthalate) composition,        thereby preparing a surface-solidified strand;    -   (c) separating the surface-solidified strand from the water;    -   (d) exposing the surface-solidified strand to ambient air in an        annealing region for a sufficient period of time to induce cold        crystallization on the surface of the strand producing a        surface-crystallized strand; and    -   (e) contacting the surface-crystallized strand with water in a        second quench region for a sufficient period of time to solidify        the interior of the strand.

The object of the process hereof is to create a radial temperaturegradient in the extruded strand in a direction normal to the directionof motion thereof (that is, normal to the longitudinal directionthereof) so that upon exiting the first quench region the surface of thestrand has solidified while at least the preponderant portion of theinterior of the strand remains above the cold crystallizationtemperature, T_(cc) “Preponderant portion” shall mean at least 50% ofthe interior of the strand. Then, in the annealing region, heat from thewarmer interior will be transferred to the cooler surface, causing thesurface to undergo crystallization. Once the surface has undergonecrystallization, the still warm strand can then be thoroughly quenchedin the second quench region so that the strand will be ready forpelletization, accumulation or other handling step.

In the art of thermal analysis, T_(g) and T_(cc) refer to specifictemperatures determined according to specified methods from, forexample, differential scanning calorimetry data. However, the averagepractitioner of the art understands that these temperatures sodetermined lie within a range of several degrees that define thetransition to which they refer. Thus, for example, cold crystallizationwill occur over a range of several degrees, but the rate, under one setof standard test conditions, reaches a maximum at T_(cc).

The critical temperatures referred to, namely the glass transitiontemperature (T_(g)) and the cold crystallization temperature (T_(cc))are known to shift somewhat depending upon the specific components ofthe PTT composition hereof. For example, T_(g) and T_(cc) are both knownto vary depending upon the type and concentration of comonomer in thepolymer.

While T_(g) and T_(cc) have clear and precise scientific definitions,they are defined phenomenologically for the purposes of the presentinvention, without reference to an actual temperature reading. T_(g) isdefined for the purposes of the present invention as that temperatureabove which polymer molecules exhibit sufficient mobility so that pelletsticking behavior can occur. T_(cc) is defined for the purposes of thepresent invention as that temperature at which the surface of the strandundergoes crystallization at a maximum rate while in the annealingregion.

It is not necessary to actually measure T_(g) and T_(cc) as hereindefined. When the molten polymer strand is extruded, it is, bydefinition, above both T_(g), T_(cc), and the crystalline melting pointof the polymeric composition. When the molten strand exits the firstquench region according to the process hereof, it is clear to thepractitioner hereof that the surface has solidified and is no longermolten. Moreover, the surface-hardened strand must have sufficientstrength to resist breakage under tension.

On the other hand, if the surface of the strand has not become at leastpartially crystallized upon exiting the annealing region, a practitionercan conclude that the residence time in the first quench region was toolong, so that the temperature of a preponderant portion of the interiorhad been reduced to below T_(cc).

Suitable for the practice of the present invention are homopolymers ofPTT and copolymers thereof comprising at least 70% of monomer units oftrimethylene terephthalate. Polymers suitable for the practice of thepresent invention can contain up to 25% by weight of various additivesas are commonly incorporated into polymers to impart additionalfunctionality. Such additives include but are not limited to UVstabilizers, inorganic fillers, antistats, pigments, and flameretardants. Any suitable PTT composition must exhibit coldcrystallization in order to be operable in the process hereof.

In one embodiment, the PTT composition comprises PTT homopolymer.

The strand according to the invention is a moving strand. That is, anyarbitrarily selected point on the strand undergoes continuoustranslation away from the strand die in a direction parallel to thelongitudinal axis of the strand. Taken in its entirety at any givenmoment in time, the strand extends from the die to the pelletizer.However, for the purposes of the present invention the term “strand”shall also be understood to refer to any appropriate segment of theextruded strand, of any length. For example, when the text refers tocontacting the strand with water, it shall be understood that at anygiven moment in time, a segment of the strand, shorter in length thanthe entire strand, is undergoing water contact.

For the purposes of the present invention, the term “residence time”shall refer to that period of time in which an arbitrarily selectedpoint along the strand spends traversing a particular region as definedherein. The term “the residence time of the strand” shall be understoodto mean the residence time of an arbitrary point along the strand.

In one embodiment, the extrudate consists of a single strand. In analternative embodiment, the extrudate comprises a plurality of strands.

According to the present invention, the residence time of the strand inthe first quench region is sufficient to reduce the temperature of thesurface to a temperature where the surface is solid and has sufficientstrength to resist breakage upon tension, but insufficient to cause atleast a preponderant portion of the interior to fall below the coldcrystallization temperature of the poly(trimethylene terephthalate)composition. In one embodiment, the residence time of the strand in thefirst quench region lies in the range of 0.2 to 2 sec. In a furtherembodiment, the residence time of the strand in the first quench regionlies in the range of 0.3 to 1.0 sec.

Further according to the invention, the strand exits the first quenchregion and enters the annealing region where the surface undergoesreheating at least to T_(cc) by the latent heat remaining in theinterior of the strand. According to the invention, the residence timeof the strand in the annealing region is sufficient to inducecrystallization on the surface of the strand. In one embodiment, theresidence time of the strand in the annealing region lies in the rangeof 0.3 to 5 sec. In a further embodiment, the residence time of thestrand in the annealing region lies in the range of 0.5 to 2.5 sec.

Further according to the invention, following the annealing region, thestrand is passed through a second water quench region for a period oftime sufficient to further solidify the interior of the strand. In oneembodiment, the residence time in the second quench region lies in therange of 1 to 10 sec. In a further embodiment the residence time in thesecond quench region lies in the range of 1.5 to 3.5 sec.

In one embodiment, the water temperature, T_(q), in the first and secondquench regions is, independently, in the range of 0° C.<T_(w)<60° C. Ina further embodiment, 20° C.≦T_(w)≦50° C. In a still further embodiment,35° C.≦T_(w)≦45° C.

The water quench may be effected by any means known to be acceptable inthe art. In one embodiment, water quenching is effected by immersing thestrand in a water bath. In an alternative embodiment, the waterquenching is effected by passing the strand through a water spray. Inone embodiment of the process hereof, the water quench is effected bysame means in both the first and second quench regions. In analternative embodiment, the water quench is effected by different meansin the first and second quench regions.

As the strand exits the first quench region and enters the annealingregion, it must be separated from residual quench water residing on orentrained with the strand surface. Any convenient means for soseparating is satisfactory, including air jet drying, draining throughdrain holes, passing over a dessicant bed, and the like. While it is notnecessary for the strand to be bone dry, it is necessary for the propercrystallization of the surface to replace the high heat capacity liquidwater on the strand surface with low heat capacity air.

Following quenching in the second quench region, the strand can befurther processed. In an embodiment, the strand is fed to a pelletizerfor cutting into pellets that are well-suited for production of fiber,film, and injection molded parts, among other uses. In an alternativeembodiment, the strand itself may be collected in bulk.

An apparatus is described for use with the present process wherein theapparatus comprises

-   -   an extrusion means for extruding a molten polymer strand;    -   a pelletizer; and,    -   a trough disposed to convey a polymer strand from the extrusion        means to the pelletizer; the trough comprising        -   an interior bottom surface;        -   a water dispensing means disposed to provide a layer of            water on a first portion of the interior bottom surface that            is proximate to the extrusion means;        -   a mesh or perforated surface disposed in the trough            downstream from the water dispensing means, disposed to            permit the separation of water from a water-immersed strand            incident upon it; and,        -   a second portion of the interior bottom surface downstream            from that mesh or perforated surface, disposed to contain a            layer of water.

The extrusion means is not critical so long as it results in theextrusion of a molten strand. Suitable means include screw-typeextruders, Farrell continuous mixers, gear pumps, so-called continuouspolymerizers, screw pumps, hydraulic pumps and the like. Any of theseextrusion means is employed herein to feed a strand die from which canbe extruded a molten strand, according to the invention.

Any pelletizer designed for pelletizing polymer strands is suitable foruse in the present invention. Pelletizers are well known in the art, andare widely available commercially from many sources.

The trough is disposed to define a path between the strand die and thepelletizer, conveying the strand from strand die to pelletizer. For thepurposes herein, the term “downstream” refers to the relative positionwith respect to the path from the strand die to the pelletizer. Forexample, a second position along the path is further downstream than afirst position if the second position is further from the strand die andcloser to the pelletizer than is the first position. The trough providesthe means for quenching the strand according to the process of theinvention.

A suitable trough can be made of any water impermeable material,including but not limited to metal, ceramic, or plastic. In a typicalembodiment hereof, the trough is made from metal, typically stainlesssteel or aluminum. The trough is designed to hold a layer of water, intowhich the strand is immersed according to the process of the invention.

The trough comprises a water dispensing means that is disposed todispense water into a first portion of the interior bottom surface ofthe trough where that first portion is proximate to, and generally asclose as possible, to the strand die so that the molten strands can beimmersed in as short a time as possible after exiting the strand die.Suitable water dispensing means can be any that are known in the art.Suitable water dispensing means can be water sprays, water pumps, hosesor tubes any of which can be connected to a water tap or reservoir.

The trough further comprises a mesh or perforated surface downstreamfrom the first portion of the trough, the mesh or perforated surfacedisposed to separate the water from a water immersed strand that isexiting the first portion of the trough, according to the processhereof. In one embodiment, the mesh can be any material that forms amesh, grid, or netting that allows water to pass through. It istypically made of a non-water absorbing material such as metal, ceramic,or plastic.

In an alternative embodiment, the perforated surface comprises aplurality of perforations in the interior bottom surface of the troughitself. In a further embodiment, the trough further comprises a seriesof cover plates that can be used to cover the perforations on theexterior bottom surface of the trough, thereby adjusting the length ofthe perforated surface.

In either embodiment, the water entrained by the strand in the processis able to drain off through the perforations or mesh, allowing thesegment of the strand traversing this portion to undergo annealing inair according to the process hereof.

The trough further comprises a second portion of the interior bottomsurface that is disposed to contain a layer of water, this for the finalquench in the process hereof. In one embodiment, wherein the mesh orperforated surface comprises a raised structure that resides above thewater level, only a single water dispensing means is necessary since thewater can flow freely underneath the elevated mesh or perforatedsurface. In an alternative embodiment, wherein the perforated surface isa portion of the interior bottom surface itself, the water input in thefirst portion proximate to the strand die will drain out the holes inthe bottom of the trough, unless it is saved by inserting a system ofbaffles and gutters to direct the water to the second portion,downstream from the mesh or perforated surface. Alternatively, the waterrequired for the second portion can be supplied by a second waterdispensing means that can be any of those suitable for the first waterdispensing means.

FIG. 1a is a side longitudinal cross-sectional schematic view of anembodiment wherein the trough comprises a plurality of perforations inthe interior bottom surface thereof. The metal trough, 1, is disposed sothat water, 2, dispensed from a first dispensing means, 3, forms a layerflowing downstream, and then draining through a plurality of drainholes,4, on the interior bottom surface of the trough (not seen in this view).A second water dispensing means, 5, is disposed to dispense water, 6, toform a second water layer flowing downstream to a drainage hole, 7, (notseen in this view) in the downstream end of the trough.

FIG. 1b is a view of the exterior bottom, 8, of the trough shown in FIG.1a , showing the plurality of perforations, 4, downstream from the firstportion, and the larger drainage hole, 7, in the furthest downstreamportion.

FIG. 2 is a side longitudinal cross-sectional schematic view of thetrough, 21, in an embodiment wherein the trough interior bottom does nothave perforations downstream from the first portion of the trough sothat water, 22, dispensed by a water dispensing means, 23, introduced atthe portion closest to the strand die can flow unimpeded to the drainagehole, 24, at the end of the trough. Instead, in this embodiment, thetrough is provided with a raised wire mesh portion, 25, that is disposedjust above the water.

The invention is further described in but not limited to the followingspecific embodiments.

EXAMPLES

FIG. 3 is a schematic depiction of the apparatus in the Examples hereof.A K-Tron® Loss in Weight feeder, 31, was used to feed pellets, 32, ofDuPont Sorona® J1141 (1.02 dL/g IV, available from the DuPont Company)to the feed throat, 33, of a 40 mm Werner and Pfleiderer co-rotatingtwin screw extruder, 34, at a rate of 296 lbs/hour. In the extruder thepellets were melted and the resulting melt conveyed to an 8-hole stranddie, 35, from which 8 strands, 36, were extruded (only one shown),moving at 93 cm/sec. The 40 mm extruder was characterized by a barrellength of 1780 mm and L/D=45. The extrusion conditions are shown inTable 1:

TABLE 1 Set Point Actual Feed Barrel 1 Cold water Barrell 2 220° C. 221°C. Barrell 3 220° C. 221° C. Barrell 4 220° C. 244° C. Barrell 5 220° C.233° C. Barrell 6 220° C. 238° C. Barrell 7 220° C. 240° C. Barrell 8220° C. 258° C. Barrell 9 220° C. 254° C. Barrell 10 220° C. 255° C.Barrell 11 220° C. 219° C. Die 220° C. 227° C. Screw speed  395 RPM DiePressure  290 psi Vacuum 25.0 in. H₂O Feeder speed  293 lb/hr Extrudatemelt 269° C.

Within about 3 inches from exiting the die, 35, the molten strand, 36,was directed using guides (not shown) through the first quench region,37, by immersion in 11° C. water, 38, dispensed from a water shower, 39,contained in a 12 foot long water trough, 310. The strand exited thefirst quench region, 37, into the annealing region, 311, and wasconveyed over a wire mesh, 312, elevated above the water, 38. Followingthe annealing region, 311, the strand was passed into the second quenchregion, 313, using guides (not shown) from which the quenched strand,314, was fed to a pelletizer, 315, while the water drained out by adrain hole, 316 (not shown). The pelletizer, 315, chopped the strandinto pellets, 317, about ⅛ inch in length, and were collected in acollection can, 318.

The residence time of the strand in the annealing region was varied bymaking small adjustments in the length of the wire mesh, 312. Theresidence time was calculated from the calculated rate of speed of thestrand based on mass flow, pellet diameter and density and the actualdistance traversed by the strand.

The densities of the pellets prepared under each set of conditions weredetermined from He pycnometry.

A 23 g aliquot of pellets from each test condition was placed in a 7 cmdiameter aluminum pan. A sheet of aluminum foil was placed over the pan,and the thus covered pan was placed in a temperature-controlledhydraulic press at 40° C. and 18.5 kPa for 24 hours. After 24 hours, thepan was removed and the pellets that had aggregated to one another wereremoved by hand, and weighed. The percentage of the weight of theoriginal 23 g aliquot that had aggregated was recorded as the “%sticking.”

Comparative Example A represents the standard practice in the artadapted from the process for pelletizing PET.

Comparative Example B represents a condition in which the residence timein the annealing region was insufficient.

TABLE 2 Residence Times (sec) First Quench Annealing Density StickingRegion Region (g/cc) (%) CE A all underwater 0 1.308 42 CE B 0.35 1.081.310 43 Ex 1 0.35 1.22 1.312 17 Ex 2 0.35 2.19 1.314 2 Ex 3 0.35 2.511.337 0

We claim:
 1. A process comprising: (a) preparing a molten strand of apoly(trimethylene terephthalate) composition wherein the strand has asurface and an interior; (b) contacting the molten strand with water ina first quench region for a sufficient period of time so that thesurface of the molten strand solidifies and at least a preponderantportion of the interior remains above the cold crystallizationtemperature of the poly(trimethylene terephthalate) composition, therebypreparing a surface-solidified strand; (c) separating thesurface-solidified strand from the water; (d) exposing thesurface-solidified strand to ambient air in an annealing region for asufficient period of time to induce cold crystallization on the surfaceof the strand producing a surface-crystallized strand; and (e)contacting the surface-crystallized strand with water in a second quenchregion for a sufficient period of time to solidify the interior of thestrand.
 2. The process of claim 1 further comprising pelletizing thestrand after it exits the second quench region.
 3. The process of claim1 wherein the period of time in the first quench region lies in therange of 0.2 to 2 seconds.
 4. The process of claim 3 wherein the periodof time in the first quench region lies in the range of 0.3 to 1.0second.
 5. The process of claim 1 wherein the period of time in theannealing region lies in the range of 0.3 to 5 seconds.
 6. The processof claim 5 wherein the period of time in the annealing region lies inthe range of 0.5 to 2.5 seconds.
 7. The process of claim 1 wherein theperiod of time in the second quench region lies in the range of 1 to 10seconds.
 8. The process of claim 7 wherein the period of time in thesecond quench region lies in the range of 1.5 to 3.5 seconds.
 9. Theprocess of claim 1 wherein the water in the first and second quenchregions, independently, has a temperature of greater than 0° C. but lessthan 60° C.
 10. The process of claim 9 wherein the water in the firstand second quench regions, independently, has a temperature within therange of 35 to 45° C.
 11. The process of claim 1 wherein the period oftime in the first quench region lies in the range of 0.3 to 1.0 second;the period of time in the annealing region lies in the range of 0.5 to2.5 seconds; the period of time in the second quench region lies in therange of 1.5 to 3.5 seconds; the water in the first and second quenchregions, independently, has a temperature in the region of 35 to 45° C.